1. Field of the Invention
The present invention relates to a frequency division multiplex transmission signal receiving apparatus, and specifically relates to an improvement of a reception technique where multipath, fading or the like occurs in received signals, deteriorating the demodulation capability.
2. Description of Related Art
In recent years, digital transmission of audio and video signals has actively been developed, and OFDM has been employed in the broadcasting and telecommunications field as an optimum system. In OFDM, data is allocated to a plurality of carriers that are orthogonal to each other, and modulated and demodulated. On the sender side, IFFT (Inverse Fast Fourier Transform) is performed, and on the receiver side, FFT (Fast Fourier Transform) is performed.
For each carrier, an arbitrary modulation method can be used, and QAM transmission using synchronous detection or transmission using delay detection can be employed. In synchronous detection, a pilot signal is periodically inserted, and on the receiver side, a deviation from the pilot signal is obtained to perform amplitude and phase equalization.
Alternatively, differential encoding is performed between symbols, and received signals are demodulated by means of delay detection, without being subjected to carrier reproduction.
In digital transmission, error correction is essential from the perspective of deterioration caused by transmission paths, and transmission characteristic enhancement.
Methods have been proposed (see, for example, Japanese Patent No. 3872950) responding to the case where although Gaussian noise-optimized Viterbi decoding is performed, noise distribution is different from the Gaussian distribution because of the existence of a fading transmission path, which causes sharp time variations, and reflected waves called multipath, resulting in impossibility to provide sufficient performance.
However, only with individual parameters such as frequency variation and time variation, which are used in Japanese Patent No. 3872950, optimum correction adapted to differences arising from complex reception conditions cannot be made.